Functional patterns of thumb key pinch and their influence on thumb strength and stability

Abstract

Introduction Trapeziometacarpal osteoarthritis (TMC OA) is a common condition frequently related to instability and hypermobility. Many factors are involved in the development of this degenerative process. Recent research suggests that the position of the metacarpophalangeal (MCP) joint may influence TMC joint contact wear patterns and subsequent development of OA. The aim of this study is to explore the alignment of the MCP joint of the thumb during lateral pinch in order to describe the naturally occurring alignment in a group of healthy volunteers without symptoms. Methods One hundred and nine participants were asked to apply a lateral pinch manoeuvre to a pinchmeter, while their thumbs were assessed. Passive hypermobility of the thumb MCP (MCP) joint was assessed, key pinch strength was measured, key pinch position was photographed and the angle of the joints during the assessment was measured. Results Four different functional pinch patterns were identified. The most frequent pattern seems to be MCP hyperextension described by Moulton as harmful for the TMC joint. The level of strength obtained did not correspond to the pinch pattern used, and hypermobility of the thumb MCP joint did not determine the position of the thumb during key pinch. Conclusion Four different thumb postures were identified during lateral pinch; however, these postures were not associated with any difference in pinch strength. We conclude that the more unstable and mechanically inefficient postures are compensated by functional capacity.